Vehicles, such as a riding lawn mowers, with “zero turning radius” (ZTR) capability are well-known in the art. Many existing ZTR riding lawn mower designs use two individually controlled pumps, each for independently controlling a drive wheel motor to achieve extreme ZTR maneuverability characteristics. This is referred to as “drive-wheel-steering” because the steering is controlled by the rotational ratio of the left and right drive tires, with the tires at the other end of the chassis being mounted for free rotation, such as on casters. Navigating this type of vehicle generally requires a skilled operator, since the controls are usually in the form of a non-intuitive joystick or twin steering levers associated with the separate pumps. The same problem exists in bulldozers and many other types of vehicles where crawler tracks provide not only the motive force for the vehicle, but also control the direction in which the vehicle travels.
Another ZTR control arrangement requires two different pumps mounted in the same transmission, as recently introduced by John Deere in the form of a SPIN-STEER mower and described in U.S. Pat. No. 6,257,357B1. In this arrangement, one pump is dedicated to chassis propulsion while the other is dedicated to steering the chassis. Although this design is capable of utilizing a more convenient steering wheel instead of steering levers, it deleteriously requires complicated linkages. Additionally, the vehicle using this control arrangement has a noticeably variable turn rate that depends upon chassis speed. It also has drive line packaging limitations due to the provision of the dual pumps in an all-in-one integral transmission. Since many commercial mowers utilize drive trains with individually mounted components such as wheel motors, pumps, etc., use of this control arrangement is generally unsuitable, absent substantial design modifications.
Accordingly, a need exists for eliminating the complexity associated with this and similar prior art proposals, while at the same time improving the overall operation of the vehicle having ZTR capability.